1. Field of the Invention
The present invention relates in general to an organic electroluminescent device and, more particularly, to an organic electroluminescent device capable of emitting visible phosphorescence and emitting visible fluorescence at the same time.
2. Description of the Related Art
A conventional organic electroluminescent device (OELD) is a multi-layer structure including a substrate, an anode, a cathode, a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer and an emissive layer. The anode, the hole injection layer, the hole transport layer, the emissive layer, the electron transport layer, the electron injection layer and the cathode are disposed on the substrate orderly from bottom to top. The emissive layer includes a host-dopant system, in which a few dopants are doped in host material. How to define the host-dopant system to be a fluorescent host-dopant system or a phosphorescent host-dopant system is illustrated as follows.
When a voltage is applied to the cathode and the anode, electrons are injected into the emissive layer from the cathode through the electron injection layer and the electron transport layer. Holes are injected into the emissive layer from the anode through the hole injection layer and the hole transport layer. After the electrons and the holes combine in the emissive layer, the host material is excited from a ground state to an excited state. Because the host material in the excited state is unstable, the host material certainly returns to the ground state and transfer energy to the dopants.
When the dopants receiving the energy are excited from the ground state to the excited state, singlet excitons and triplet excitons are generated by the dopants. In both the fluorescent dopants and the phosphorescent dopants, due to the distribution ratio of the electron spin state, the probability of forming the triplet excitons and the singlet excitons is approximately 3:1.
Electroluminescence occurs in the organic electroluminescent device while the singlet excitons or the triplet excitons return to the ground state by releasing photons. In the fluorescent host-dopant system, only the singlet excitons emit visible fluorescence when returning to the ground state. In the phosphorescent host-dopant system, when returning to the ground state, the triplet excitons emit visible phosphorescence and the singlet excitons emit light which can be transferred to phosphorescence through internal system crossing (ISC).
In the fluorescent host-dopant system, the exciton lifetime of the singlet excitons when returning to the ground state from the excited state is approximately rated in nanoseconds (ns), and the visible fluorescence is emitted during the exciton lifetime.
In the phosphorescent host-dopant system, the exciton lifetime of the triplet excitons when returning to the ground state from the excited state is rated approximately in microseconds (μs), and the visible phosphorescence is emitted during the exciton lifetime. In the organic electroluminescence mechanism, the internal quantum efficiency of the phosphorescent dopants is approximately 4 times the internal quantum efficiency of the fluorescent dopants (the theoretical value can be 100%) due to the following two reason: (1) the distribution ratio of the electron spin state results in the probability of forming the triplet excitons and the singlet excitons to be 3:1; and (2) the phosphorescent dopants can transfer the energy from the singlet excitons of the host material to the triplet excitons of the dopants. In other words, 25% of light emitted from the organic electroluminescent device is fluorescence in the singlet state, and 75% of the light emitted from the organic electroluminescent device is phosphorescence in the triplet state. In the phosphorescent dopants, the singlet excitons are transferred to the triplet excitons to enable the internal quantum efficiency to be 100%. Therefore, the luminance efficiency of the organic electroluminescent device having the phosphorescent host-dopant system (such as a phosphorescent device) is better than the luminance efficiency of the organic, electroluminescent device having the fluorescent host-dopant system (such as a fluorescent device).
However, the long exciton lifetime is a disadvantage of the phosphorescent host-dopant system. The exciton lifetime of the triplet excitons is rated in microseconds (μs); that is, the time when the triplet excitons stay in the emissive layer is approximately 1000 times the time of the singlet excitons. Therefore, the long staying time of the triplet excitons in the emissive layer results in triplet-triplet annihilation. In other words, one triplet exciton in the excited state tends to collide with another triplet in the excited state, resulting in the energy of the two triplet excitons lost in the form of heat or vibration instead of releasing photons. Therefore, the luminance efficiency of the organic electroluminescent device having the phosphorescent host-dopant system (such as phosphorescent device) declines rapidly as the electric current increases.